1. To determine by light and electron microscopy, whether the fibroblast-like reticular cells which we found characteristic of the stroma of only granuloid spleen colonies of mice, and whose processes make intimate contact with adjacent myeloblasts, have the same plasma-membrane-associated Alalkine phosphatase reactivity as the fibroblast-type reticulum cells whose processes Westen and Bainton found closely associated with myeloblasts in rat and mouse bone marrow. 2. To determine whether the dendritic reticular cells characteristic of the stroma of only erythroid spleen colonies, and whose processes are intimately associated with the surrounding proerythroblasts, have the same abundant lysosomal acid phosphatase reactivity as the reticulum cells Westen and Bainton found to be associated with erythroid precursors in bone marrow. 3. To determine whether the dendritic reticular cells of erythroid spleen colonies are also present in the presumptive-erythroid spleen colonies of polycythemic mice, and whether their processes make contact with the newly differentiating erythropoietin-sensitive stem cells of which such colonies are composed. 4. To isolate erythroblastic nests (EN) from erythropoietically stimulated mouse spleens or from fetal liver and separate them into their central nurse cell (NC) and surrounding erythroblasts (EBI) by the methods of Macario. To study the ultrastructure and histochemistry of the NC and its relation to the cells described in 2 and 3 above, and to surrounding EBI. To study the interaction of NC with EBI regarding induction of responsiveness to erythropoietin (EPO) and maturation to red blood cells (RBC). To determine whether NC in irradiation chimeras are of host or donor origin. 5. To determine if the erythropoietic stromal defect of the SI/SId mouse is a defect of NC. Long-term, we hope to be able to test whether NC are responsible for the induction of EPO-receptors. These studies have significance for control of differentiation of pluripotent stem cells, and the etiology of blood dyscrasias.